Biggest-ever black holes found

Kate Taylor, 6th December 2011

Astronomers have discovered the two largest black holes ever - 10 billion times as massive as our own sun - which look set to consume everything within a region five times the size of our solar system.

The black holes are at the centers of two galaxies more than 300 million light years from Earth, and may be the remnants of quasars, the bright galaxies that populated the early universe.

"In the early universe, there were lots of quasars or active galactic nuclei, and some were expected to be powered by black holes as big as 10 billion solar masses or more," says Chung-Pei Ma, UC Berkeley professor of astronomy.

"These two new supermassive black holes are similar in mass to young quasars, and may be the missing link between quasars and the supermassive black holes we see today."

Around 63 supermassive black holes have been found so far,in the cores of nearby galaxies. The largest for more than three decades was a 6.3 billion solar mass black hole in the center of the nearby galaxy M87.

However, one of the newly discovered black holes, located 320 million light years away in the elliptical galaxy NGC 3842, weighs in at 9.7 billion solar masses.

The second, in the elliptical galaxy NGC 4889, is at least as large.

According to UC Berkeley graduate student Nicholas McConnell, these black holes have an event horizon 200 times the orbit of Earth. Beyond the event horizon, each has a gravitational influence extending over a sphere 4,000 light years across.

"For comparison, these black holes are 2,500 times as massive as the black hole at the center of the Milky Way Galaxy, whose event horizon is one fifth the orbit of Mercury," he says.
During their active quasar days some 10 billion years ago, these black holes would have cleared out the neighborhood by swallowing vast quantities of gas and dust.

Astronomers believe that many, if not all, galaxies have a massive black hole at the center, with the larger galaxies harboring larger black holes.

The black holes were detected using telescopes at the Gemini and Keck observatories in Hawaii and at McDonald Observatory in Texas. The team obtained detailed spectra of the diffuse starlight at the centers of several massive elliptical galaxies.

Having such huge masses contained within a volume only a few hundred light years across implied that they were massive black holes.

"If all that mass were in stars, then we would see their light," says Ma.